Nitrate-ester plasticizers are commonly used in commercial and military explosives and propellants. For example, glyceryl trinitrate (nitroglycerine, or simply NG) is used in dynamite and in double-base propellants, and triethylene glycol dinitrate (TEGDN) is used in tactical propellant systems. Nitrate esters are conventionally produced by a nitration reaction of a polyol with a so-called “mixed acid,” which includes nitric acid (HNO3) and sulfuric acid (H2SO4). See, e.g., J. P. Agrawal and R. D. Hodgson, Organic Chemistry of Explosives, pp. 90 and 91 (2007). The mixed acid contains nitrating species, such as nitronium ions (—NO2+) which react with hydroxyl groups on a polyol starting material, substituting a nitro group for the hydrogen in an O-nitration reaction. The mixed acid nitration reaction is used to synthesize NG from the polyol glycerin (glycerol) and TEGDN from the polyol triethylene glycol (TEG). These mixed acid nitration reactions may be performed in a Biazzi nitrator or other continuous-type configuration, such as a pipe nitrator. Mixed acid nitration reactions and reaction equipment are described in Military Explosives, Dept. of the Army Technical Manual, TM No. 9-1300-214, pp. 8-10 to 8-13 (1984). The mixed acid nitration reaction utilizes an extended washing train system to extract the nitrate ester. Nitration may also be conducted utilizing only nitric acid, which is referred to herein as a nitric acid nitration reaction.
Other nitrate esters may be formed by similar mechanisms. For example, ethylene glycol dinitrate (EGDN or nitroglycol), diethylene glycol dinitrate (DEGDN), diglycerol tetranitrate (DGTN), metriol trinitrate (MTN), 1,2,4-butanetriol trinitrate (BTTN), 1,2-propanediol dinitrate (PDDN), trimethylolethane trinitrate (TMETN), and pentaerythritol tetranitrate (PETN) may be formed by the mixed acid nitration reaction of ethylene glycol, diethylene glycol, diglycerol, methyltrimethylolmethane (metriol), 1,2,4-butanetriol (BT), 1,2-propanediol, 1,1,1-tris(hydroxymethyl)-ethane (THME), and pentaerythritol, respectively.
However, mixed acid nitration reactions have a number of disadvantages. Because the nitration reactions are exothermic, special care must be, and is, taken to avoid thermal runaway and explosions, especially in large-scale processes. In addition, the high solubility and thermal instability of the nitrate esters in the mixed acid lead to poor separation of the nitrate ester. Furthermore, dissolved nitrate esters in the waste acid may react with nitric acid, resulting in violent oxidative fume-off. An extended water washing train may be used to limit these problems. However, additional processing equipment may be required to safely remove or destroy the nitrate ester residue present in the waste water streams.
Furthermore, oxidative side reactions and fume-off of nitric acid may reduce the concentration of the active nitrating agent in mixed acid nitrations. Therefore, excess nitric acid may be required for such reactions. In the case of nitric acid nitration reactions, the molar ratio of HNO3 to —OH is essentially infinite since the nitric acid also serves as the solvent.
Because of the difficulty and expense of processing excess reagents and large volumes of hazardous waste streams, some nitrate esters are not in production domestically. For example, the Naval Surface Warfare Center, Indian Head Division, and Copperhead Chemical have both stopped current production of TEGDN. The complexity of the required equipment makes production of some nitrate esters economically unattractive, leading to sporadic commercial availability of these compounds.
Other methods of nitrating compounds are also known. Carbon atoms of some aromatic compounds may be nitrated using a mixture of a nitrate salt and sulfuric acid, as described in U.S. Pat. No. 7,737,308, which is assigned to the Assignee of the present application. A polycyclic nitramine, such as 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.05,903,11] dodecane (TEX), may be formed by nitration of nitrogen atoms on a cyclic hydrocarbon using ammonium nitrate as a nitrate source, as described in U.S. Pat. No. 5,529,649, which is assigned to the Assignee of the present application.